FWIW, these will fit on other Infinitis. They pretty much all use the same lug pattern. So if they don't sell here in a timeframe you're happy with, you might post them on another forum.
Oh, and for those considering, this sounds like a pretty good price to me. If you came to my dealership and tried to order these, you'd pay a whole lot more.
These would be great for someone planning to have a winter and a summer set of wheels who didn't get 20's with their JX. Use your stock tires on the 18's this winter and next summer until they're nearly burned out. Then put snows on the 18's for the following winter and buy tires for these 20" wheels for the summer after that.
As of the last time I checked, there are no winter tires (real winter tires, I'm not talking M+S tires) for the 20" rims, but there are plenty of snow tire options for the 18" rims.